1. Field of the Invention
The present invention generally relates to the manufacture of electronic components and, more particularly, to the improvement of adhesion between a metallization layer and an underlying polymer insulating layer included therein.
2. Description of the Prior Art
As functions demanded from integrated circuits has increased in recent years, it has become the practice to increase production yields of electronic devices through the use of packaging of integrated circuits in modules. So-called multi-layer modules (MLM), including structures commonly referred to as multi-layer ceramic (MLC) modules, are exemplary of such packaging types. These modules allow the inclusion of a plurality of integrated circuit chips within the same module and provide for a potentially highly complex interconnection wiring pattern between the chips. These structures also allow the inclusion of chips formed by different technologies which may be mutually exclusive, thereby allowing for increased flexibility of circuit design.
In order to make desired contacts to the chips included therein, such modular structures usually include so-called distribution wiring which accommodates the extremely small regions on a chip where desired connections are to be made. The feature sizes encountered in distribution wiring is many times smaller and densities many times greater than in the interconnection wiring used in the remainder of the module. The distribution wiring will, therefore, typically be formed from a plurality of layers of polymer (e.g. polyimide) insulator having selected areas coated with a metal. Connections between layers are achieved by depositing metal in small holes, called vias, in the respective polyimide layers.
At the present state of the art, it is considered cost-effective to test and repair such modules at particular points during the course of manufacture. One such type of repair can include the removal and replacement of one or more of the chips of the module. As is known in the art, the connection from the distribution wiring to a chip is accomplished by the use of a pad termination or formation of the distribution wiring and the flow of solder between that pad and a corresponding pad on the chip. Solder is often provided as a preform, such as in the so-called controlled collapse chip connection type of pad (hereinafter referred to as simply a C4 pad), and the preform is reflowed to form the connection. Therefore, during either soldering or desoldering of a chip, substantial heat and mechanical stress is applied to such a connection pad. Since the module or at least the chip is heated substantially uniformly during such a process, the remainder of the metallization is subjected to heat stress and to some degree of mechanical stress due to the differential dimensional changes with temperature of the metallization and the polymer insulator. Accordingly, since soldering and desoldering may be required a number of times adhesion between the metallization and the polymer insulator becomes extremely important if the production of further defects in the metallization is to be avoided.
It should also be noted that pads and other portions of the metallization pattern will traverse metal-filled vias, as alluded to above. At the point where the metallization overlies such a structure, also commonly referred to as a connection stud, metal-to-metal bonding will be provided or occur inherently by virtue of the particular deposition process used. This bonding effectively enhances the adherence of the metallization to the substrate although the actual adherence to the polymer is not, itself, enhanced. However, in many cases of pattern design, it is not possible or desirable to form such a stud beneath all pads or even at a sufficient number of points to significantly reduce the tendency toward separation of the metallization from the underlying polymer material. Such regions which are not formed over such studs are referred to as being "unanchored". Accordingly, the tendency toward separation of metallization from an underlying polymer over the course of numerous applications of thermal and/or mechanical stress, as are typically encountered during the manufacture, including testing and repair, of such modules, cannot be adequately controlled by the mere multiplication of such studs.
While the design of modular packages generally includes provision for repair of broken conductors, it must be appreciated that loss of adhesion may not immediately result in the breaking of a connection. In fact, such a loss of adhesion may not be initially detectable by electrical testing at all but may represent a latent defect which will break and cause malfunctioning of the module after it is completed and put in service. When a loss of adhesion occurs, normal thermal cycling of the module during normal operation may cause flexing of the connection sufficient to cause metal fatigue and fracture of a conductor if the conductor is not mechanically supported by adhesion to a polymer layer. In such a case, the manufacture is likely to have at least progressed beyond the point where repair is possible, thus, at least, decreasing manufacturing yields of the modules and increasing economic cost. It is also likely that economic cost will be further increased if failure occurs during field operation since testing and diagnosis of the entire unit in which the module is installed will often be required in addition to the economic cost of down-time of the unit.
It is to be understood that the problem of adhesion of metal to polymer dielectrics, and polyimide dielectrics in particular, is not unique to the manufacture of multi-layer electronic circuit modules but may be encountered in other technologies involved in other types of electronic elements such as the formation of capacitors or even other technologies entirely unrelated to the fabrication of electrical devices. However, since the problems of loss of adhesion are of substantial economic importance at present in the fabrication of multi-layer modules, the present disclosure will be directed thereto in order to convey an understanding of the meritorious effects of the invention in comparison with the prior art.